This invention relates to inductive loop presence detectors, and more particularly inductive loop installations for detecting the presence of vehicles.
When more than one inductive loop vehicle detector is used on a typical installation the detectors can cause mutual interference between one another commonly called "cross talk" . The effect is usually brought about by inductive coupling between the loops laid in the roadway although it can be produced by capacitive coupling.
Each road loop is connected to an oscillator and when "cross talk" occurs frequency sidebands are produced in the oscillator whose amplitude and frequency depend upon the frequency separation between the interfering oscillators. This can disturb the sensing circuitry that follows each oscillator and the purpose of which is normally to identify changes in frequency of the loop oscillator caused by vehicle movements.